The present invention relates to a liquid crystal display device and, more particularly, to an improvement in a field effect liquid crystal display device for time-multiplexed driving.
A conventional so-called twisted nematic liquid crystal display device has a 90.degree. twisted helical structure of a nematic liquid crystal having positive dielectric anisotropy and sealed between two substrates having transparent electrodes arranged thereon in desired display patterns as described in G.B. Pat. No. 1,372,868. Polarizing plates are arranged on outer surfaces of the substrates such that polarizing or absorption axes thereof become perpendicular or parallel to the major axes of the liquid crystal molecules adjacent to the substrates.
In order to twist the liquid crystal molecules between the two substrates through 90.degree., orienting layers are formed on said electrodes and exposed surfaces of the substrates by coating polyimide resin and making numerous fine grooves by rubbing the coated surfaces which contact the liquid crystal molecules by a cloth along one direction. In this case, the major axes of the liquid crystal molecules adjacent to the surface become parallel to this one direction, i.e., a rubbing direction. Two rubbed surfaces are spaced apart so as to oppose each other while their rubbing directions are crossed at about 90.degree.. These rubbed substrates are then sealed with a sealing agent, and a nematic liquid crystal having positive dielectric anisotropy is filled in a space formed between the substrates. Therefore, the major axes of the liquid crystal molecules are twisted through about 90.degree. between the substrates. The resultant liquid crystal cell is sandwiched between a pair of polarizing plates with their polarizing or absorption axes substantially parallel to the major axes of liquid crystal molecules adjacent thereto, respectively. In a conventional reflective type liquid crystal display device which is most frequently used, a reflector is disposed on the outer surface of the lower polarizing plate. Light incident on the upper surface of the device is linearly polarized by the polarizing plate or polarizer. In a portion of a liquid crystal layer which is not applied with a voltage, the plane of polarization of the linearly polarized light is rotated through 90.degree. along the helical structure and is transmitted through the lower polarizing plate. The light is then reflected by the reflector and returns to the upper surface of the device. However, in a portion of the liquid crystal layer which is applied with a voltage, where the helical structure is destroyed, the plane of polarization of the linearly polarized light will not be rotated. Therefore, the linearly polarized light transmitted through the upper polarizing plate is blocked by the lower polarizing plate and will not reach the reflector. In this manner, electrical signals can be converted into optical images in accordance with the presence or absence of an electrical potential applied across the liquid crystal layer.
The twisted nematic type liquid crystal display device (hereinafter referred to as "TN-LCD" for short), owing to its merits such as low driving voltage, low power consumption, small thickness, and light weight, has found extensive utility in wrist watches, desk-top computers, various industrial measuring instruments, and automotive instruments.
The dot matrix type TN-LCD which is capable of displaying letters and figures has long been arousing much interest as useful for display devices in portable computers and various data terminals. At present 64.times.480 and 128.times.480 dot matrix display devices multiplexed at a 1/64 duty factor are on the market. The demand in market, however, is shifting to LCD's with still higher contents of display and information density such as those of 200.times.640 picture elements and 256.times.640 picture elements which are equivalent in display capacity to cathode-ray picture tubes. For such LCD's to be commercially feasible, they are required to be effectively driven in a highly time-multiplexed fashion of the order of duty factor of 1/100 or 1/128.
FIG. 1 is a graph showing typical luminance-voltage characteristics of a conventional reflective type liquid crystal display device having a 90.degree. twisted helical structure of a nematic liquid crystal and with axes of its polarizers intersecting at right angles. The graph shows the relative luminance of reflected light as a function of the applied voltage. An initial value of luminance is taken as 100% with no votage applied, and a final value when little or no further change in luminance occurs at sufficiently high voltage is taken as 0%. In practice, a pixel is sufficiently bright when the relative luminance is more than 80%, so that the pixel is considered to be in an OFF state, and when the relative luminance is less than 20%, the pixel is dark enough for further decrease in luminance to be imperceptable to the eye, and hence the pixel is considered to be in an ON state. Voltages corresponding to 80% and 20% of relative luminances are given as the threshold voltage Vth and the saturation voltage Vsat, respectively, hereinafter. In other words, the threshold voltage Vth is given as a maximum allowable voltage corresponding to the OFF state, and the saturation voltage Vsat is given as a minimum allowable voltage corresponding to the ON state. For a transmissive type liquid crystal display device, FIG. 1 would represent transmission-voltage characteristics.
The electrooptical characteristics of the liquid crystal display device change in accordance with a viewing angle. These characteristics limit a viewing angle range within which a good display quality is obtained. A viewing angle .phi. will be defined with reference to FIG. 2. Referring to FIG. 2, in a liquid crystal display device 1, a nematic liquid crystal 33 having positive dielectric anisotropy is sandwiched between two substrates 11, 12 having transparent electrodes arranged thereon in desired display patterns and orienting layers (not shown) which are formed on the electrodes and exposed surfaces of the substrates by coating, for example, polyimide resin and making numerous fine grooves by rubbing the coated surfaces with a cloth unidirectionally. A rubbing direction of an upper substrate 11 of a liquid crystal display device 1 is represented by reference numeral 2, a rubbing direction of a lower substrate 12 is represented by reference numeral 3, and a twist angle between major axes of liquid crystal molecules adjacent to the upper substrate and those of liquid crystal molecules adjacent to the lower substrate is represented by 4.
X- and Y-axes are located on the surface of the liquid crystal display device 1. The X-axis defines a direction for bisecting the twist angle 4 of the liquid crystal molecules. A Z-axis defines a normal to the X-Y plane. An angle between a viewing direction 5 and the Z-axis is defined as the viewing angle .phi.. In this case, by way of simplicity, the viewing direction 5 is in the X-Z plane. The viewing angle .phi. in FIG. 2 is regarded to be positive. Since contrast becomes high when viewed from a direction in the X-Z plane, this direction is called the viewing direction 5.
Performance parameters for a quantification of time-multiplexed driving characteristics in the subsequent description will be briefly described below.
FIG. 1 is a graph showing typical luminance-voltage characteristics of a reflective twisted nematic type liquid crystal display device when its polarizing axes are crossed. Commercially available conventional liquid crystal display devices have acceptable viewing angles falling within a range of 10.degree. to 40.degree.. At a viewing angle .phi. of 10.degree., a driving voltage giving 80% luminance, at which liquid crystal display device begins to appear to be "on" to an observer is designated by Vth1, and a driving voltage giving 20% luminance below which further decrease in luminance begins to be almost imperceptable to the eye is designated by Vsat1, and at a viewing angle .phi. of 40.degree., a driving voltage giving 80% luminance is designated by Vth2.
The sharpness of the luminance-voltage characteristic curve, .gamma., the viewing-angle dependence of luminance, .DELTA..phi., and the time-multiplexability, m are defined as follows: EQU .gamma.=Vsat1/Vth1 EQU .DELTA..phi.=Vth2/Vth1 EQU m=Vth2/Vsat1
Assuming luminance-voltage characteristic curves are ideal, the two curves at different viewing angles .phi. of 10.degree. and 40.degree. coincide, and the curves are steep enough for both a threshold voltage and a saturation voltage to have the same value.
The time-multiplexed driving characteristics of the conventional liquid crystal display device depends on .DELTA.n.d where .DELTA.n is the refractive index anisotropy, i.e., optical anisotropy of the liquid crystal and d is the distance between the upper and lower substrates. When .DELTA.n.d is large (e.g., more than 0.8 .mu.m), the sharpness of the luminance-voltage characteristic, .gamma. becomes good (small value), and the viewing-angle dependence, .DELTA..phi. is poor (small value). However, when .DELTA.n.d is small (e.g., less than 0.8 .mu.m), the sharpness of the luminance-voltage characteristic, .gamma. becomes poor (large value) and the viewing-angle dependence, .DELTA..phi. becomes good (large value). However, the time-multiplexability, m (=.DELTA..phi./.gamma.) is about the same irrespective of .DELTA.n.d. Two typical examples are shown in Table 1.
TABLE 1 ______________________________________ Performance .DELTA.n .multidot. d Parameters 0.5 .mu.m 1.0 .mu.m ______________________________________ .gamma. 1.237 1.148 .DELTA..phi. 0.938 0.874 m 0.758 0.761 ______________________________________
Time-multiplexed driving will be briefly described with reference to a dot matrix display. As shown in FIG. 3, Y stripe electrodes (signal electrodes) 13 and X stripe electrodes (scanning electrodes) 14 are formed on the lower and upper substrates 12, 11 (not shown), respectively. Pixels (picture elements), liquid crystal portions at intersections of the X and Y electrodes 14 and 13 are chosen to be in an ON state or an OFF state so as to display characters or the like. N scanning electrodes X1, X2, . . . , Xn are successively and repeatedly scanned in the order named in a time-multiplexed manner. When a given scanning electrode, e.g., X3 in FIG. 3 is selected, a selection or nonselection display signal is simultaneously applied to all pixels P31, P32, . . . and P3m on the given scanning electrode through the signal electrodes 13 constituted by electrodes Y1, Y2, . . . and Ym in accordance with a display signal. In other words, the ON or OFF state of the pixels at the intersections of the scanning electrodes and the signal electrodes is determined by a combination of voltage puleses applied to the scanning and signal exectrodes. In this case, the number of scanning electrodes 14 corresponds to the number of time-multiplexing.
The conventional liquid crystal display device has poor time-multiplexed drive characteristics as shown in Table 1, and these characteristics would permit practical time-multiplexing of only a maximum of 32 or 64. However, demand has arisen to improve the image quality of the liquid crystal display device and increase information content to be displayed. Any conventional liquid crystal display devices cannot satisfy these needs.